The present invention relates to undervoltage protection for power distribution circuits feeding voltage sensitive loads, such as motors.
Typically undervoltage protection is provided by a so-called "undervoltage release" (UVR) device, such as a solenoid whose operating coil is connected to be energized by the distribution circuit voltage. If the system voltage remains above a predetermined minimum level, the solenoid coil is sufficiently energized to magnetically hold in the solenoid plunger against the bias of a calibrated retracting spring. When the system voltage falls below the predetermined minimum level, the retracting spring overpowers the diminished magnetic attractive forces developed by the solenoid coil, and the plunger is retracted. In the process, the plunger strikes a latch, tripping a circuit breaker to interrupt the distribution circuit. When the system voltage is returned to normal, the magnetic attractive forces developed by the UVR solenoid coil are not sufficient to pull the plunger in. Consequently, the plunger must be reset mechanically, usually coincidentally with the opening movement of the breaker contact or the resetting of the breaker operating mechanism preparatory to reclosure of the breaker contacts.
The principal disadvantage to this straightforward approach to undervoltage protection is the inability of the undervoltage release solenoid to hold in for harmless, momentary dips in the system voltage or momentary losses of system power. Unless a mechanical dash-potting arrangement is provided, the UVR solenoid plunger will, under these circumstances, be pulled out by its retracting spring and the circuit breaker needlessly tripped. To overcome this disadvantage undervoltage protection control circuits have been provided for developing a DC control voltage proportional to the system voltage which is then utilized to hold a UVR solenoid in as long as the system voltage remains above the pre-established minimum level. An energy storage capacitor, charged by the DC control voltage, is connected across the UVR solenoid coil. During momentary dips or losses of system voltage, the energy storage capacitor discharges through the UVR solenoid coil to hold it in. Using this approach, UVR solenoid drop-out delays of up to eight seconds can be readily achieved.
The utilization of an undervoltage protection control circuit lends practicability to an electrical approach, rather than a mechanical approach, to resetting the UVR solenoid after it has operated to trip the circuit breaker. One such approach is to apply the DC control voltage across the UVR solenoid coil and a series voltage dropping resistor, such that the voltage across the UVR coil is approximately one-half the control voltage but nevertheless sufficient to hold the UVR solenoid plunger in. The dropping resistor is shunted by a large electrolytic capacitor. Upon restoration of nominal system voltage and then DC control voltage after the UVR solenoid has dropped out, the electrolytic capacitor effectively bypasses the dropping resistor to apply full control voltage across the UVR solenoid coil which is sufficient excitation to magnetically pull the plunger in to its seated, reset position against the bias of its retracting spring. As this electrolytic capacitor charges up, the voltage across the dropping resistor rises exponentially nd the voltage across the UVR solenoid falls exponentially toward their respective normal operating levels.
While this UVR solenoid resetting approach is simple and straightforward, it is not without problems. Full control voltage is applied to the UVR solenoid coil only briefly, i.e., while the resetting capacitor is fully discharged. This limits the maximum pull-in force to a very brief, initial period, and it decreases as the square of the exponentially decreasing capacitor charging current. Consequently, the electrical resetting of the UVR solenoid can be a tenuous proposition. Moreover, the electrolytic resetting capacitor is prone to lose its dielectric properties if left inactive for long periods of time. Under this circumstance, the sudden application of control voltage can precipitate its failure. Furthermore, an electrolytic capacitor is a large, excessively space consuming component which presents serious packaging problems.
It is accordingly an object of the present invention to provide an improved undervoltage protection control circuit for power distribution systems.
A further object is to provide an undervoltage protection control circuit of the above character wherein electrical resetting of a UVR solenoid is effected in an efficient, reliable manner.
An additional object is to provide an undervoltage protection control circuit of the above character which is capable of providing varying, protracted UVR solenoid drop-out delays.
Yet another object is to provide an undervoltage protection control circuit of the above character which is equipped to further accommodate shunt tripping of a circuit breaker via the UVR solenoid.
A still further object of the present invention is to provide an undervoltage protection control circuit of the above character which is inexpensive to manufacture, efficient and reliable in operation, and compact in size.
Other objects of the invention will in part be obvious and in part appear hereinafter.